1. Field of the Invention
The present invention relates to a method for producing a trialkyl gallium for use as an ingredient for forming a compound semiconductor thin film such as GaN by epitaxial crystal growth by MOCVD (metalorganic chemical vapor deposition) and like techniques.
2. Description of the Related Art
With the advancement of cellular phones and optical communication technologies, demand is rapidly growing for compound semiconductors for use in high speed electronic devices such as high electron mobility transistors (HEMTs) and heterojunction bipolar transistors (HBTs) for use in cellular phones; semiconductor lasers for use with optical communications, DVDs, etc.; optical devices such as white and blue super high-intensity LEDs for use in displays; and other applications.
In general, alkyl metals containing group IIB (group 12) and group IIIB (group 13) elements, and in particular methyl or ethyl metals, are often used as metalorganics (MOs) for use as ingredients of compound semiconductors. A great demand exists for, in particular, group IIIB gallium in the form of alkylgalliums for the production of compound semiconductors by MOCVD with group VB (group 15) elements such as nitrogen, arsenic, and the like.
A typical example of a prior-art method for producing an alkylgallium is reacting an alkyl halide with a gallium-magnesium mixture or gallium-magnesium alloy. This method is advantageous in allowing commercially readily available metallic gallium and metallic magnesium of high purity to be used as received, and in not requiring the use of a reagent for which care must be taken.
Some examples of similar methods are given below. Many prior-art methods use a gallium-magnesium alloy because the use of a gallium-magnesium alloy as a starting material results in the production of a trialkyl gallium in higher yields than the use of a gallium-magnesium mixture. U.S. Pat. No. 5,248,800 describes that the use of a gallium-magnesium alloy having a molar ratio of Mg/Ga from 1.6 to 2.4 in the reaction of a gallium-magnesium alloy and an alkyl iodide results in the production of trialkyl gallium in a high yield of 80 to 90%. Furthermore, U.S. Pat. No. 5,248,800 discloses that the yield of a trialkyl gallium with the use of a gallium-magnesium mixture is 15%. That is, this patent publication clearly states that a method that uses a gallium-magnesium alloy as a starting material produces a trialkyl gallium in a higher yield than the method using a gallium-magnesium mixture.
UK Patent No. 2123423 discloses a method for producing a trialkyl gallium in which a gallium-magnesium alloy and an alkyl iodide are reacted in the presence of an ether.
Methods using a gallium-magnesium alloy require a process of preparing an alloy by heating, and it is difficult to prepare a uniform gallium-magnesium alloy. Reports state that constant yields cannot be obtained due to this difficulty (A. C. Jones, D. J. Cole-Hamilton, A. K. Holliday, M. J. Mahmad, J. Chem. Soc., Dalton Trans., 1047 (1983); K. B. Starowieski, K. J. Klabunde, Appl. Organomet. Chem., 3, 219 (1989)).
Therefore, with an eye to the simplification of the production process and stable productivity, it is desirable to use a gallium-magnesium mixture, which is simpler.
In this connection, L. I. Zakharkin, V. V. Gavrilenko, N. P. Fatyushina, Russ. Chem. Bull., 46, 379 (1997) discloses a method in which trimethyl gallium is directly produced by co-pulverizing a gallium-magnesium-methyl iodide mixture while heating. Moreover, this publication discloses a method for producing triethyl gallium in which a gallium-magnesium mixture and a small amount of iodine are vacuum-heated, and then reacted with ethyl iodide in the presence of hexane or in the absence of a solvent. The yield of triethyl gallium is substantially the same when a gallium-magnesium alloy or gallium-magnesium mixture is used, or when vacuum heating is conducted in the presence of hexane or in the absence of a solvent. In this publication, powdered magnesium is used.
V. I. Bregadze, L. M. Golubinskaya, B. I. Kozyrkin, J. Clust. Sci., 13, 631 (2002) discloses that trimethyl gallium can be obtained in a high yield of 80 to 90% by reacting a gallium-magnesium mixture and methyl iodide in the presence of isoamyl ether. In this publication, powdered magnesium is also used.
The use of a gallium-magnesium mixture usually results in a lower yield than the use of a gallium-magnesium alloy. Attempts to obtain a high yield using a gallium-magnesium mixture limit the usable form of magnesium to powders as described in the aforementioned Russ. Chem. Bull., 46, 379 (1997); and J. Clust. Sci., 13, 631 (2002).
The majority of such prior-art methods use alkyl iodides, which are most reactive among the alkyl halides. Although an alkyl iodide has an advantage, i.e., high reactivity, it is likely to result in a Wurtz coupling reaction as a side reaction as shown in Reaction Formula (1) below:2RI+Mg→R—R+MgI2   (1)wherein R is an alkyl group.
Moreover, alkyl iodides are more expensive than alkyl bromides and alkyl chlorides. In addition, since the boiling point of the resulting trimethyl gallium and the boiling point of methyl iodide are close, it is difficult to isolate the trimethyl gallium for purification.
With an eye to negating such disadvantages of alkyl iodides, Japanese Unexamined Patent Publication No. 1991-123784 discloses the use of a methyl bromide and methyl iodide mixture as an alkyl halide in a method for producing trimethyl gallium in which a gallium-magnesium alloy and a methyl halide are reacted in order to reduce the total amount of methyl iodide necessary in the reaction.
Methyl bromide has a higher reactivity than methyl chloride. However, methyl bromide has been designated as an ozone-depleting substance, and the use thereof has been gradually reduced since 1999. Hence, alkyl iodides and alkyl bromides have, as described above, both advantages and disadvantages.
It is presumed that the reaction that produces trialkyl gallium, i.e., the reaction of gallium, magnesium, and alkyl halide, follows Reaction Formula (2) below:2Ga+aMg+(a+3)RX→2GaR3+3MgX2+(a−3)RMgX   (2)wherein R is an alkyl group, X is a halogen atom, and a is a positive integer.
It can be deduced that an alkyl magnesium halide (RMgX), i.e., a Grignard reagent, is involved in the production of the trialkyl gallium.
The Synthesis, Reactions and Structures of Organometallic Compounds, in Comprehensive Organometallic Chemistry, S. G. Wilkinson, F. G. A. Stones, E. W. Abel, Eds.; Vol. 1, Pergamon Press Ltd., (1982) Chapter 4 teaches that the production of a Grignard reagent is greatly affected by the type of halogen contained in an alkyl halide and the type of reaction solvent, and that the extent of halogen reactivity is, from strongest to weakest, iodine>bromine>chlorine>fluorine.
This publication teaches that Lewis bases such as ethers and amines are usually used as reaction solvents and such solvents are likely to form adducts with trialkyl galliums, and the use of hydrocarbons as reaction solvents that barely form adducts with trialkyl galliums significantly impairs the Grignard reagent productivity.
Moreover, M. J. S. Gynane, I. J. Worral, J. Organomet. Chem., 40, C59 (1972) discloses that the reactivity of alkyl halides to gallium is significantly weaker than to magnesium. This publication teaches that the extent of reactivity of alkyl halides to gallium, as with their reactivity to magnesium, is, from strongest to weakest, alkyl iodide>alkyl bromide. Although it does not refer to the reactivity of alkyl chlorides, it is presumable that the reactivity of alkyl chlorides is weaker still.
Soviet Patent Publication No. 388563 discloses a method in which an alkyl chloride is used as an alkylating agent. In this publication, a trialkyl gallium is produced by reacting an alkyl halide with a gallium-magnesium mixture or gallium-magnesium alloy in the presence of a Lewis base such as dibutyl ether, diisoamyl ether, diethyl ether, or like ether. However, the reaction using an alkyl chloride does not produce a trialkyl gallium in a satisfactory yield. No reaction of a gallium-magnesium mixture and an alkyl chloride is described in this patent publication.
Although alkyl chlorides do not have the disadvantages of alkyl iodides and alkyl bromides, alkyl chlorides are very poorly reactive with respect to gallium and magnesium. So far, no alkyl halide satisfies all requirements.
The methods described in the aforementioned publications mostly use ethers as reaction solvents. Use of ethers results in enhanced reactivity and yield; however, ethers coordinate with the resulting alkyl gallium, thereby forming an adduct. Depending on the type of ether, it is often difficult to separate ether adducts from alkyl galliums even when distillation and like techniques are used, making it difficult to produce metalorganic compounds of high purity. The use of a metalorganic compound containing ether adducts thereof as an ingredient in the production of compound semiconductors by MOCVD tends to impair the electric properties of the resulting compound semiconductors due to the presence of oxygen during crystal growth. Hence, use of ethers as solvents has, as described above, both advantages and disadvantages
The aforementioned Russ. Chem. Bull., 46, 379 (1997) discloses a method that does not use an ether. Although this method does not use any ether at all, purification of a trialkyl gallium to a high purity may be difficult due to the wear on grinders and grinding balls if a starting mixture is ground prior to the production of a trialkyl gallium or due to the iodine contamination if iodine is used.
Japanese Unexamined Patent Publication No. 1989-301684 discloses that, with respect to a method for producing a trialkyl gallium using a gallium-magnesium alloy, a trialkyl gallium can be produced using an ether as a reaction solvent in a less than stoichiometric amount relative to magnesium.
Japanese Unexamined Patent Publication No. 1991-127795 teaches that, with respect to a method for producing a trialkyl gallium using a gallium-magnesium alloy, a trialkyl gallium can be produced using as reaction solvents a hydrocarbon in combination with an ether in a less than stoichiometric amount relative to magnesium.
As stated above, there are prior-art methods that use hydrocarbons as reaction solvents. Hydrocarbon solvents do not have the disadvantages described above in connection with the ether solvents. However, the use of hydrocarbon solvents usually results in low reactivity and thus production of a trialkyl gallium in a poor yield, as described in the aforementioned The Synthesis, Reactions and Structures of Organometallic Compounds, in Comprehensive Organometallic Chemistry, S. G. Wilkinson, F. G. A. Stones, E. W. Abel, Eds.; Vol. 1, Pergamon Press Ltd., (1982), Chapter 4.
Ethers and hydrocarbons have their advantages and disadvantages as described above. So far, no solvent meets all requirements.
It is preferable to use a hydrocarbon as a solvent and an alkyl chloride as an alkyl halide to allow the resulting trialkyl gallium to be highly purified. However, the use of these compounds results in poor reactivity and yields.
The prior-art methods for producing a trialkyl gallium described above use the reaction of an alkyl halide with a gallium-magnesium mixture or gallium-magnesium alloy. Other prior-art methods for producing an alkyl gallium include (a) to (e) below:    (a) reaction of a gallium halide with a Grignard reagent (for example, U.S. Pat. No. 4,604,473);    (b) reaction of a gallium halide with a trialkyl aluminum (for example, K. K. Fukin, I. A. Frolov, Tr. Khim. Khim. Tekhnol., 4, 40 (1973));    (c) reaction of a gallium halide with an alkyl lithium (for example, R. A. Kovar, H. Derr, D. Brandau, J. O. Calloway, Inorg. Chem., 14, 2809 (1975));    (d) reaction of a gallium halide with a dialkyl zinc (for example, C. A. Kraus, F. E. Toonder, Proc. Natl. Acad. Sci. USA, 19, 292 (1933)); and    (e) reaction of gallium and a dialkylmercury (for example, G. E. Coates, J. Chem. Soc., 2003 (1951)).
In these methods, a gallium halide or elemental gallium is used as the gallium-containing ingredient. There is currently no method known for efficiently producing a trialkyl gallium using other gallium-containing ingredients.